The present invention relates to an engine control apparatus for controlling an engine (internal combustion engine) installed on an automobile or the like, and in particular to an engine control apparatus which can ensure and maintain stable and constant emission characteristics even in such a condition that a fuel nature (as heavy fuel, light fuel or the like) is unsure, and which is robust among different fuel natures.
These years, since the regulations of emission control as to automobile engines have been more and more tightened in the North America, Europe, Japan and the like, there has been the demand of further reducing engine exhaust emission. The performance of catalyst and the degree of precision of the catalyst control have been enhanced, and accordingly, the volume of exhaust emission from an engine is dominant during a start of the engine. In general, natures of fuel are uneven in a predetermined range, and accordingly, fuel evaporation rates vary, depending upon their natures during low temperature operation. Since the fuel combustion amount varies with respect to a fuel injection quantity, the difference among fuel natures affect upon the starting characteristic and the exhaust performance (exhaust emission characteristics) of an engine. Thus, as disclosed in, for example, JP-A-8-177553 and JP-A-2001-41094, there have been heretofore proposed various systems in which the engine control is adapted to a nature of fuel which is used at present in the engine.
Namely, the JP-A-8-177553 discloses an engine control apparatus comprising a means for calculating a heavy fuel quantity, a means for calculating a light fuel quantity, a means for determining a fuel nature, and a means for changing over from the heavy fuel calculating means into the light fuel calculating means when it is determined that a fuel to be used is a light fuel. However, since the engine control apparatus disclosed in this document is initially set for a heavy fuel, a relatively large quantity of fuel is injected until the use of light fuel is confirmed, and accordingly, the air-fuel ratio becomes rich, resulting in deterioration of emission characteristics. Further, in addition to the above-mentioned configuration, although a system for determining a fuel nature in view of a variation in engine speed is used, should a light fuel be used under setting of a heavy fuel, no substantial difference in engine speed would be appreciated when the air-fuel ratio is changed over into the rich side even though the air-fuel ratio become richer than that during the use of a heavy fuel, as shown in FIG. 13. This is because toque (pressure in cylinder) does not vary appreciably on a side richer than the theoretical air-fuel ratio. Thus, it is fundamentally difficult to determine a fuel nature from an engine speed, and should the use of a light fuel be unable to be determined, fuel injection would be carried out under setting of a heavy fuel, deterioration of emission characteristics is unavoidable.
Further, the JP-A-2001-41094 discloses an engine control apparatus comprising a first start control means for a light fuel, a second start control means for a heavy fuel, for changing over from the first start control means into the second start control means if a start is not completed within a predetermined time. However, since the engine control apparatus disclosed in this document, is initially set for a light weight fuel, the starting performance deteriorates in the case of using a heavy fuel, and accordingly, in the worst case, the product value, that is, the reliability would be detrimentally affected. Further, should the evaporation rate of the heavy fuel be extremely low, no initial detonation would occur, and accordingly, unburnt injected fuel would be emitted into the atmosphere, resulting in a problem of deterioration of emission characteristics.